Low-frequency amplifier



Oct. 10, 1950 w. A. ANDERSON LOW-FREQUENCY AMPLIFIER Filed April 18,1946 INVENTOR WARREN A. ANDERSON ATTO RN EY indicated at +32.

i atentecl i0,

LOW-FREQUENCY AMPLIi I'fifi Warren A. Anderson, West Brighton, N. Y assignor to Radio Corporation of America, a corporation of DelawareApplication April 18, 1946, Serial No. 662,978

3 Claims.

This invention relates to low frequency power amplifiers, and moreparticularly to improvements which would stabilize and improve theoperation of such amplifiers. a

In carrying out my invention I have utilized the inherently high plateresistance of a pentode discharge tube as an impedance to couple to aload the output energy from a second discharge tube operating as a lowfrequency amplifier. A combination of several resistors and a capacitoris used to stabilize and improve the operation of the amplifier circuitincluding the coupling impedance.

From the foregoing it will be understood that the principal object of myinvention is to stabilize and improve the operation of a low frequencypower amplifier. Other objects and advantages of my invention will bemade apparent in the description to follow. This description isaccompanied by a drawing the sole figure of which represents a preferredcircuit arrangement.

Referring to the drawing, I show therein two pentode discharge tubes V1and V2. In the tube V1 is an anode I connected to a main direct currentsource the positive terminal of which is The cathode 2 and thesuppressor grid 5 are connected through a resistor R1 to an anode 6 intube V2. This tube also possesses a cathode l, a control grid 8, ascreen grid 9 and a suppressor grid Ill. The cathode 'l and suppressorgrid II] are connected through a resistor R2 to ground which may beconsidered th negative terminalof the main direct current source.

Suitable screen grid potential may be obtained from taps on the maindirect current source or from a voltage divider which is connectedacross the terminals of this source. For convenience, however, thescreen grid 4 is shown connected to the positive terminal of anindependent source II the negative terminal of which is connected to theanode 6 of tube V2. The screen grid 9 is indicated as being connected tothe positive source terminal B1 which is of lower voltage than that ofthe terminal B2.

Input signals are applied to terminals l2, one of which is grounded andthe other of which is coupled across a capacitor [3 to the control gridin tube V2. The potentials of the input signals :are designated em.Output signals are designated em and are impressed across any suitableload .or utilization device indicated by the component marked Z. Thisload is coupled to the output .circuit of tube V2 across a capacitor [4.

The control grid 8 in tube V2 is connected to ground through a resistorI5. The control grid 3 in tube V1 is suitably biased by means of avoltage divider consisting of resistors R3, R4 and R5, where a tap onresistor R4 constitutes this element as a potentiometer and the tapitself is connected to the grid 3 through a resistor R6. The grid 3 isalso coupled through capacitor C1 to the anode 6 in tube V2.

Cathode resistors R1 and R2 are included to increase the linearity ofthe characteristics of tubes V1 and V2, respectively. In some cases,however, these resistors may be omitted. The choice of a pentode typetube V1 as the coupling impedance may be attributed to the inherentlyhigh plate resistance of such a tube and to its relatively low impedanceto direct current. In this way a minimum of direct current power iswasted in the coupling impedance and a maximum of alternating power isdissipated in the load impedance Z. On the other hand, if the plateresistance of a triode could be made sufiiciently high with respect tothe load impedance Z, say, by a factor of 10, then equally satisfactoryoperation might be expected.

Since the distribution of the plate supply voltage between tubes V1 andV2 is all important when operating tubes in series, and since minorvariations in tube characteristics are unavoidable, particularly in thecase of pentodes, wide variations in this distribution may becompensated by the use of stabilizing resistors R3. R4, R5 and Re andcapacitor C1. The resistors maintain the grid of the coupling impedancetube at a constant D.-C. potential. Thus, if any variation in the platevoltage distribution between the tubes occurs, it automatically producesa change in the grid-to-cathode voltage of the coupling tube which tendsto compensate for the original change in the voltage distribution.Condenser C1 causes the grid of the coupling tube to follow the signalexcursions of its cathode and therefore to behave in its normal fashion.

I claim:

1. An electronic amplifier comprising two discharge devices each havinga cathode, an anode and three grids, namely a control grid, a screengrid and a suppressor grid, means including a direct current source andtwo circuits connected across its terminals for activating said devices,one of said circuits being arranged to traverse the space paths of saiddevices in series, the other of said circuits constituting a voltagedivider, said divider having an adjustable tap thereon connected to thecontrol grid of the device at the positive end of the series circuit,means appropriate to each device for positively biasing its screen gridwith respect to its cathode, the cathode and suppressor rid in eachdevice being interconnected, means for applying input signals to thecontrol grid of said device at the negative end of the series circuit,and means coupling a load to the anode and cathode of the last saiddevice.

2. An amplifier according to claim 1 and including a capacitor incircuit between the grid of the device at the positive end of theaforesaid series circuit and the anode of the other device.

3. In a low frequency power amplifier, two electron discharge deviceseach having electrodes including at least an anode, a cathode, a controlgrid,a screen grid and a suppressor grid connected to the cathode, inputterminals on which the low frequency voltage to be amplified may beimpressed, a coupling between said input terminals and the control gridand cathode of one device, a source of direct potential having anegative terminalconnected to the cathode of said one device and apositive terminal connected to the anode of the other device, a resistorconnecting the anode of said one device to the cathode of the said otherdevice, an output circuit coupled across the anode to cathode impedanceof said one device, a biasing circuit including a resistor connectedbetween the control grid and cathode of said one device, means for.maintaining the screen grids of said devices electropositive relative totheir cathodes, apparatus for applying a substantially constant biasingpotential to the control grid of the other device comprising a voltagedivider in parallel with said source of direct potential, said voltagedivider including a potentiometer resistor with a point thereonconnected through a voltage dropping resistor to the control grid ofsaid other device, and means ap plying to the control grid of said otherdevice a potential which follows the potential set up on the cathode ofsaid other device by virtue of its connections to the anode of the onedevice, comprising a capacitor coupling the anode of said one device tothe control grid of the other device.

WARREN A. ANDERSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,305,919 Eaton Dec. 22, 19422,326,614 Bowman Aug. 10, 1943. 2,428,295 Scantlebury Sept. 30, 1947FOREIGN PATENTS Number Country Date 540,834 Great Britain Oct. 31,1941

